This invention relates to processes and apparatus concerning heat and/or mass transfer between a gas and a solid or a liquid.
Reference will be made hereinbelow to drying processes and apparatus but it is to be understood that the invention has application to other heat and mass transfer processes.
In the treatment of various materials, drying can be an important process, whether it forms the entire process or merely one stage of the treatment. Drying involves the transfer of heat to the material being dried and the transfer of mass (vapour) from the material.
It is known to enhance heat and/or mass transfer processes by the use of standing transverse and longitudinal waves, for example in pulsed columns and in pulsed combustion systems. Such processes are of limited application, however, and often offer little significant advantage. They also frequently give rise to undesirable sound emission.
GB 1150406 describes a fish slurry drying process in which tangentially arranged jet engines inject hot gas in pulses creating a poorly understood flow regime in a drying tank. It is suggested that a net flow of hot gas is subjected to oscillations in each engine exhaust leading to scrubbing of wet particles injected into the exhaust. Flow in the drying tank may involve a series of mutually concentric cyclone-like vortexes alternating upwardly and downwardly in the tank.
The present invention provides a heat and/or mass transfer process and apparatus wherein the materials to be treated are subjected to a travelling tangential wave during their passage through the apparatus.
According to a first aspect of the invention there is provided a heat and/or mass transfer process in which a gas is caused to impinge upon a material, the gas velocity having a component tangential to the flow direction of the material such that a fluctuating velocity is superimposed upon the mean velocity of the material in the flow direction, said velocity fluctuations being effected by means of a travelling tangential wave whilst a steady overall gas flow is maintained.
In another aspect, the invention provides an apparatus for performing a heat and/or mass transfer process which comprises a chamber within which a material can be located and within which the material can be caused to flow, and means for impinging a gas on the material located within the chamber, the means providing the gas with a velocity having a component tangential to the flow direction of the material in the chamber, such that in use a fluctuating velocity is superimposed upon the mean velocity of the material in the flow direction, and wherein the means for impinging the gas on the material located in the chamber is adapted to create a travelling tangential wave in the material.
By making use of a fluctuating velocity superimposed on the mean velocity, it has been found that the effective thermal and/or mass transfer resistance of the boundary layer can be reduced.
Moreover, by employing a travelling tangential wave there are no pressure fluctuations on the axis and this is preferably where the exhaust for the apparatus is located. In such an apparatus, the emission of sound, which is a problem with pulsating flow devices, can be substantially eliminated.
The invention is preferably applied to drying processes and apparatus, but it is understood that it is not limited thereto and, for example, it may be applied to other heating processes and apparatus, for example heating ovens, and also to mass transfer processes and apparatus, in for example scrubbing towers.
Although the material can be either solid or liquid the invention finds particular application in the treatment of particulate solid materials.